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First, we have a gear wheel that is quite different from the other gear wheels. Notice that the hole in the middle of the wheel is not like a plus, but a circle.

As a result, the wheel does not rotate when the axle, to which it is attached, rotates.

If we take a look at the inner side of the gear wheel, we will see it has four ridges.

This allows us to stack it on top of this part:

It also has four ridges and when rotating, the gear wheel on top of it will rotate as well.

If we take a closer look at this part, we will notice that its hole is not typical for an axle.

Usually, this part is used together with:

They stack together and engage perfectly. So, if we have the following construction and we rotate the axle, the red driving ring will also rotate. As the driving ring and the gear wheel are not stacked together, when we rotate the axle, the gear wheel will not rotate.

Nevertheless, if we slightly change the setup to:

by rotating the axle, we will also rotate the gear wheel.

Basically, this is the mechanism of a cluch. When the driving ring and the gearwheel are engaged, the motion from the motor is transfered to the end of the system. If they are not, the motion from the motor is not transfered further.

Finally, we have the following part:

It is used to engage and disengage the clutch. It can be achieved as follows:

A perfect example of the clutch is the catapult. When the clutch is engaged, the motor stalls and holds the brick up and the load down. Once the clutch is disengaged, there is nothing to hold the brick up. Due to its wheight, the brick goes down and the catapult shoots. As simple as that :)

Building instructions for the LEGO Mindstorms EV3 Catapult. The robot has a motor, a clutch to control it for manual shooting and a touch sensor for automatic loading. The bottom base is removed which means the Catapult could not be turned around. Nevertheless, the baseless robot is small enough to be built in class.